1. Field of the Invention
The present invention relates to an image processing method and apparatus, and particularly, to an image processing apparatus such as image scanner, printer, digital copying machine, and facsimile terminal equipment, or an image processing apparatus used in connection with the image processing apparatus as above cited, and an image processing method thereof.
2. Related Background Art
Conventionally, a digital copying machine, for example, is constituted in such a way that an original is illuminated by a light source such as a tungsten-halogen lamp, the reflected light from the original is converted into electric signals using solid image sensors such as CCDs (charge coupled device), and then converted into digital signals, and after a predetermined correcting process, the resulting signals are used to form a recorded image with a recording apparatus such as laser printer, liquid crystal printer, thermal printer, ink jet printer, etc.
In such a digital copying machine, an output containing a large quantity of information is required when a colored original is copied, and so a copying machine has been recently developed, which uses a polychromatic laser beam printer provided with developer devices for a multiplicity of colors so that variable color copy can be realized.
However, the above-mentioned copying machine, which is provided with developer devices for a multiplicity of colors to form an image with variable colors, is complex in construction due to the multiplicity of developer devices provided thereon, and is expensive because a high accuracy in registration of images is required. This is also the problem with the copying machine using a polychromatic ink jet printer.
In the formation of image requiring the transfer of information in a monochromatic facsimile or the like, an original is read with photoelectric elements such as solid image sensors, and an image is formed based on thus converted electric signals in time-series. And in forming the image, it was commonly practiced that the gradation (or density level) of an original was only sensed without discriminating the color.
However, in the case where there are some colored drawings or tables in an original, a photoelectric converted signal only shows density level for different colors, and as most colors represented on the original are usually deep, density signals are often caused to be almost equal so that the colored portions can not be distinguished from each other in reproduction.
In a reproduced image, black portions of an original can be completely reproduced, but gray portions of the original and colored drawings or tables are represented only by gray, so that the distinction therebetween becomes quite difficult.
Thus, an invention intended to reproduce distinguishably a plurality of images with different colors contained in an original is disclosed in U.S. Pat. No. 4,369,461 specification.
However, in a situation where full-color originals are abundant, the above-described invention is not applicable to patterns containing the number of different colors contained in an original subject. Therefore methods of selecting limited number of colors have been proposed.
For example, they are methods of simply selecting limited colors from an upper level of histogram, as described in Computer Graphics Vol. 16 (1982), No. 3, pp. 297-307; or investigating an axis where colors are most widely distributed over the color space, dividing the color space at an intermediate value in a range of distribution with respect to the axis, thus obtaining a limited number of partial spaces, to each of which the same processing is repeated, and thereby selecting an average color in each partial space; or dividing the color space into small partial spaces, repeating the clustering of partial spaces under a certain condition, obtaining a limited number of partial spaces, and selecting a color with the highest frequency in each partial space, as mentioned in Image Optics, IE86-4 (1986, pp. 25-30), the technical report of the Institute of Electronics Information and Communication Engineers.
Thus, multicolor images can be monochromatically represented by designating different patterns to those selected colors.
However, in the above conventional examples, as different patterns are designated to selected colors, respectively, similar colors may have different patterns. Thereby, there is a disadvantage that as a full-color original may be constructed with many different patterns.
Further, in an area with moderately changing color, the boundaries of partial areas are made distinct owing to the differences of patterns, so that the information of "moderately" changing is lost. Therefore, there was also a disadvantage that pseudo contours might arise due to distinct boundaries.